At standard atmospheric pressure and temperature, mercury (Hg) is a liquid metal. Elemental mercury is not found free and abundant in the environment, but vaporous mercury can be released in significant quantities during industrial processing of other natural resources, from mercury cell chloralkali plant exhaust gases and emission gases from hazardous waste treatment plants. Although mercury has many useful applications, it is considered a bio-hazardous material as its incorporation into human beings can cause birth defects, serious injury and may result in death. It is therefore desirable to reduce the introduction or emission of mercury into the environment to ensure the safety of the public and, for industry in which mercury is a waste or by-product, to avoid costly litigation commonly referred to as toxic torts.
One source of mercury vapor is naturally occurring geothermal steam and brine which usually contains varying amounts of non-condensible gases including "sour gas" comprised primarily of gaseous hydrogen sulfide (H.sub.2 S). Industrial applications for geothermal steam and brine require that the sour gas stream be treated with an oxidation reduction ("Redox") process well known to those skilled in this art. The presence of mercury vapor in the sour gas introduces a bio-hazard problem.
The Redox processing of sour gas oxidizes H.sub.2 S and produces elemental sulfur (S). The produced sulfur attracts and chemically bonds with mercury vapor present in the sour gas. The result is production of solid sulfur typically contaminated with more than 20 parts per million (ppm) insoluble mercury. The EPA (Environmental Protection Agency) guidelines state that a mercury level in sulfur of 20 ppm or higher is hazardous, which renders such Redox sulfur unusable. The contaminated sulfur also requires expensive and difficult, hazardous waste disposal.
Mercury also contaminates natural gas-producing reservoirs at levels from as low as 8 micrograms per cubic meter (.mu.g/m.sup.3) to as high as 300 .mu.g/m.sup.3. Mercury vapor found in both sour and natural gas streams is chemically identical. It is crucial to remove mercury vapor from natural gas as it is extracted from reservoirs and before the gas is processed into usable fuel energy. Although mercury levels in naturally occurring fuel energy gas can be low, concentrations become cumulative in effect.
Mercury amalgamates with aluminum (Al) which is used in the construction of heat exchangers utilized by natural gas plants, especially on the welds of Al cold boxes. The result is severe damage to the heat exchangers in the form of stress cracking and corrosion which eventually leads to equipment failure, plant shutdowns and even fires.
Many attempts have been made to address the problem of mercury vapor contamination. For instance, early attempts included the use of mercury-containing compositions as catalysts to purify mercury vapor contaminated gases such as described in U.S. Pat. No. 3,661,509. Other approaches included pumping H.sub.2 S into sulfur-free natural gas streams in conjunction with amine compounds to bind and separate away mercury vapor such as described in U.S. Pat. No. 4,044,098. Neither approach achieved the anticipated successes.
Metal-sulfides were next put to the test such as copper-sulfide (CuS) described in U.S. Pat. No. 4,094,777 and lead sulfide (PbS) described in U.S. Pat. No. 4,206,183. Unfortunately, the by-products of such heavy metal use were just as hazardous as the mercury vapor contamination to be removed.
Further attempts to filter mercury vapor included the use of zeolite molecular sieves such as SiO.sub.2 /Al.sub.2 O.sub.3 described in U.S. Pat. No. 4,101,631 and zeolites coated with silver, gold and platinum disclosed in U.S. Pat. Nos. 5,281,259 and 5,409,522. Zeolite sieves work well as industrial scale catalysts in general, but are not particularly suited for filtering mercury vapor, and the use of precious metals for large scale mercury removal is economically unsound.
The current state-of-the-art in mercury vapor removal from gas streams is the use of filter vessels utilizing a bed of sulfur-impregnated activated carbon as described in U.S. Pat. No. 5,248,488. However, activated carbon in such applications often present certain hazards. Workers have to undertake time-consuming and costly safety measures with each bed-loading or bed replacement because activated carbon is highly flammable and activated carbon dust is toxic when inhaled. Geothermal sour gas contains oxygen (O.sub.2) along with H.sub.2 S, and the combination of O.sub.2 and flammable activated carbon can result in an exothermic reaction or simply, an explosion.
Geothermal sour gas, natural gas and petroleum gas streams are usually saturated with water vapor. Activated carbon tends to absorb this moisture. Wet activated carbon preferentially removes O.sub.2 from the work environment and its depletion can lead to hypoxia and death. Workers who must enter the filter vessel under these conditions need to take extraordinary precautions such as wearing flame-retardant suits and masks hooked up to O.sub.2 tanks.
Even after mercury is removed from a target gas, its problems and dangers remain in the spent activated carbon filters as elemental mercury and mercuric sulfide. Currently, the only practical method for removing mercury and mercuric sulfide from spent carbon is with a mercury retort. This is a very expensive procedure and is normally used only for small amounts of spent carbon or to recover mercury from other mercury-bearing items such as thermometers. As a result, spent activated carbon filters must be disposed of as hazardous waste without the practical possibilities of recycle.
Mercury contamination of effluent liquid streams such as industrial waste fluid discharges are just as serious as vaporous mercury in gas streams. Here too, the art has proposed many solutions for mercury removal such as leaching mercury-contaminated fluids with alkali-sulphide compounds (U.S. Pat. No. 4,017,369); using colloidal bituminous emulsions (U.S. Pat. No. 4,053,401); manganese nodules with occluded sulfur (U.S. Pat. No. 4,338,288); cement kiln dust (U.S. Pat. No. 4,844,815); copper sulfides (U.S. Pat. Nos. 5,245,106 & 5,350,728); iron salts (U.S. Pat. No. 5,308,500); and impregnating porous polystyrene resin with sulfur (U.S. Pat. No. 5,401,393).
The wide range of attempts and continued strife to develop a solution is moot testimony to the problem which exists in the art for a simple, yet effective method for removing mercury from a contaminated liquid stream.
It is with consideration to the above-identified drawbacks recognized in the art that this disclosure provides the following objects of the present invention.